Orica-GreenEdge’s Darryl Impey has returned a positive A and B sample for the substance Probenecid after the South African Championships in February.

The 29 year old South African wore the yellow jersey for two stages in last year’s Tour de France and had been left off of Orica-GreenEdge’s roster for the Grand Depart this Saturday in Leeds.

Impey said on his website, “I had no knowledge of Probenecid nor have I ever taken the substance knowingly in any manner. I am committed to drug-free sport and fully intend to take all steps necessary to clear myself of all suspicion.

“The notification came as a complete shock to me just days before the start of the Tour de France, particularly since anti-doping test conducted on my on 8 and 9 February, 2014 yielded no adverse results.”

Probenecid is banned by WADA as it can be used as a masking agent. It’s primary use is for gout and can inhibit the detection of certain drugs.

Pedro Delgado tested positive for Probenecid at 1988 while he was in the Yellow Jersey, but didn’t face sanctioning as it wasn’t on WADA’s banned substance at that time (although it was banned by the IOC). Stefano Garzelli tested positive for Probenecid in 2002 and received a nine month ban.

In the lead-up to the Tour de France there have been three prominent riders sidelined in the past week: Diego Ulissi (for salbutamol), Roman Kreuziger (bio passport irregularities) and now Daryl Impey.